This invention relates to a hydraulic pressure system suitable for use in a transmission such as a hydrostatic transmission commonly referred to as an HST and a hydromechanical transmission commonly referred to as an HMT and various other hydraulic machines and instruments.
Generally, the hydraulic system adopted in the abovementioned HST or HMT comprises a hydraulic pump at the input side of the system, a hydraulic motor at the output side thereof, and a pair of main hydraulic pressure lines connecting the pump and the motor in such a manner as to form a closed loop or circuit, with the pump and the motor being of a variable displacement type so that the transmission ratio is continuously variable.
In many such systems, displacement of the hydraulic pump or motor is controlled through a pilot system, wherein there is provided a servo mechanism having a hydraulic actuator to change the angle of the swash plate or the eccentricity of the cylinder barrel of the hydraulic pump or motor, with a separate auxiliary pump being additionally provided to supply the actuator with an operating hydraulic pressure. Provision of the auxiliary pump, however, poses a problem that the number of the component parts of the apparatus increases with resulting complication of the structure thereof.
To avoid the problem a shuttle valve may be provided between the previously mentioned pair of main hydraulic pressure lines to use the hydraulic pressure taken out from the higher pressure side of the circuit as a pilot pressure for controlling the displacement of the above-mentioned pump without using any auxiliary pump.
In hydraulic pressure systems of this type used as a transmission, however, at the neutral position or a position adjacent thereto the pressure in the above-mentioned two main hydraulic pressure lines is lowered to a boost pressure, so that under normal condition it becomes impossible to operate the actuator for controlling the variable displacement of the pump and the motor. In hydraulic pressure systems of this type, in order to prevent cavitation from occurring at the inlet side of the hydraulic pump, the hydraulic pressure produced by a booster pump is supplied through a booster circuit to the lower pressure side of the main hydraulic pressure circuit. The boost pressure is usually set to a level lower than 5 kg/cm.sup.2 if it is cavitation only that is to be prevented.
On the other hand, a pressure over 20 kg/cm.sup.2 is usually required for controlling the variable displacement of the hydraulic pump and the hydraulic motor. If the boost pressure is set to a level high enough to enable the above-mentioned control, the booster pump will consume an unnecessarily large amount of energy in the normal driving condition in which the pressure in either one of the previously mentioned main hydraulic pressure lines becomes high, that is, in the condition in which it is not necessary to use the boost pressure to operate the actuator for controlling the variable displacement of the hydraulic pump and the motor. This is a great loss of energy.
The object of the present invention is to solve the above problem once and for all.